We have discovered, characterized and functionally defined a novel, immune inhibitory ligand. This ligand is hematopoietically-expressed, a distant member of the B7 Ig-superfamily, and its extracellular domain bears homology to the B7 family ligand PD-L1. This molecule is designated as V-domain Immunoglobulin Suppressor of T cell Activation (VISTA). Distinct from PD-L1, expression of VISTA is exclusively within the hematopoietic compartment and is highly regulated on myeloid antigen-presenting cells (APCs). A soluble VISTA-Ig fusion protein, or VISTA expression on APCs profoundly inhibits in vitro T cell proliferation and cytokine production. A specific anti-VISTA monoclonal antibody interfered with VISTA-induced suppression of T cell responses by VISTA+ APCs in vitro. VISTA expression on myeloid suppression cells (MDSC) within the tumor microenvironment was expressed at extremely high levels, suggesting that VISTA on MDSCs likely impedes the development of tumor specific immunity by maintaining the suppressive character of the tumor microenvironment. In a murine model of bladder carcinoma, we show that an (VISTA mab that can reverse VISTA suppression, greatly reduced solid tumor growth and enhanced host survival. Due to the preliminary observation that blocking VISTA systemically enhances cell-mediated immunity, we hypothesize that in solid tumors where MDSCs appear to be immunomodulatory, (VISTA therapy will prove to be particularly effective. Unlike many of the other PD-L family members (B7-H3, H4, H6), the hematopoietic restriction (preferentially myeloid) of VISTA together with its profound suppressive activities, and its high expression on MDSCs, makes it a unique target for immune intervention in cancer. Taken together, our findings illustrate that VISTA is a functionally non-redundant, negative regulator of immunity. The Specific Aims of this proposal are: 1) Determine the Effect of aVISTA on Tumor Regression in Murine Models.; and 2) Produce human ahuman VISTA specific monoclonal antibodies. hVISTA-Ig will be used to select GigaMab(tm) mabs through subcontract with BioAtla. BioAtla has constructed and validated a human full length IgG library (GigaMab(tm) Fully Human library) one of the largest numbers of high diversity fully human antibodies expressed in mammalian cells. Using their proprietary bioinformatic analysis tools and rational design strategies, they are expected to maximize the combinatorial diversity of human immunoglobulin heavy and light chains in a custom library to ensure generation of high affinity antibodies. Therapeutic intervention of the VISTA inhibitory pathway represents a novel approach to modulate T cell- mediated immunity for the treatment of a wide variety of cancers. The first indication we will target is ovarian cancer, which kills 13,850 women per year in the US.